The invention generally relates to piezoelectric strain actuators. In particular, the present invention relates to a hydraulic compensator for a piezoelectric actuator, and more particularly to an apparatus and method for hydraulically compensating a piezoelectrically actuated high-pressure fuel injector for internal combustion engines.
It is believed that a known piezoelectric actuator is includes a ceramic structure whose axial length can change through the application of an operating voltage. It is believed that in typical applications, the axial length can change by, for example, approximately 0.12%. In a stacked configuration, it is believed that the change in the axial length is magnified as a function of the number of actuators in the piezoelectric actuator stack. Because of the nature of the piezoelectric actuator, it is believed that a voltage application results in an instantaneous expansion of the actuator and an instantaneous movement of any structure connected to the actuator. In the field of automotive technology, especially, in internal combustion engines, it is believed that there is a need for the precise opening and closing of an injector valve element for optimizing the spray and combustion of fuel. Therefore, in internal combustion engines, it is believed that piezoelectric actuators are now employed for the precise opening and closing of the injector valve element.
During operation, it is believed that the components of an internal combustion engine experience significant thermal fluctuations that result in the thermal expansion or contraction of the engine components. For example, it is believed that a fuel injector assembly includes a valve body that may expand during operation due to the heat generated by the engine. Moreover, it is believed that a valve element operating within the valve body may contract due to contact with relatively cold fuel. If a piezoelectric actuator stack is used for the opening and closing of an injector valve element, it is believed that the thermal fluctuations can result in valve element movements that can be characterized as an insufficient opening stroke, or an insufficient sealing stroke. It is believed that this is because of the low thermal expansion characteristics of the piezoelectric actuator as compared to the thermal expansion characteristics of other engine components. For example, it is believed that a piezoelectric actuator stack is capable of 30 microns of movement and that a valve element is capable of contracting 10 microns due to temperature fluctuations, in which case the piezoelectric actuator stack loses 30% of its overall movement. Therefore, it is believed that any contractions or expansions, of a valve element can have a significant effect on fuel injector operation.
It is believed that conventional methods and apparatuses that compensate for thermal changes affecting piezoelectric actuator stack operation have drawbacks in that they either only approximate the change in length, they only provide one length change compensation for the piezoelectric actuator stack, or that they only accurately approximate the change in length of the piezoelectric actuator stack for a narrow range of temperature changes.
It is believed that there is a need to provide thermal compensation that overcomes the drawbacks of conventional methods.
The present invention provides a fuel injector. The fuel injector comprises a body having a longitudinal axis, a piezoelectric actuator that has first and second ends, a needle coupled to the first end of the piezoelectric actuator, and a hydraulic compensator coupled the second end of the piezoelectric actuator. The piezoelectric actuator includes a plurality of piezoelectric elements along the axis between the first and second ends. The needle is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. And the hydraulic compensator axially positions the piezoelectric actuator with respect to the body in response to temperature variation.
The present invention also provides a method of compensating for thermal expansion or contraction of a fuel injector. The fuel injector includes a body that has a longitudinal axis, a piezoelectric actuator that has first and second ends, a needle coupled to the first end of the piezoelectric actuator, and a hydraulic compensator coupled the second end of the piezoelectric actuator. The piezoelectric actuator includes a plurality of piezoelectric elements along the axis between the first and second ends. The needle is movable between a first configuration permitting fuel injection and a second configuration preventing fuel injection. The method comprises providing fuel from a fuel supply to the fuel injector; and axially adjusting the piezoelectric actuator with respect to the body in response to temperature variation. The axially adjusting includes moving hydraulic oil through an orifice connecting the first and second reservoirs.